1. unintentional, unforeseen. Accidental,casual,fortuitous all describe something outside the usual course of events. Accidental implies occurring unexpectedly or by chance: an accidental blow.Casual describes a passing event of slight importance: a casual reference.Fortuitous is applied to events occurring without known cause, often of a fortunate or favorable nature: a fortuitous shower of meteors. It often also implies good luck or good fortune: a fortuitous choice leading to rapid advancement.

late 14c., "non-essential," from Old French accidentel or directly from Medieval Latin accidentalis, from Latin accidentem (see accident). Meaning "outside the normal course of nature" is from early 15c.; that of "coming by chance" is from 1570s.

n.

late 14c., "non-essential quality," from accidental (adj.). The musical sense is from 1868.